An LED consists of a semiconductor junction, which emits light due to a current flowing through the junction. A white LED is typically made by using a blue or ultraviolet LED die, and adding a plastic coat to it, the coat containing a phosphor. The phosphor is used to convert the blue or ultraviolet light emitted by the LED die to a spectrum of light that more or less closely resembles white light or blackbody radiation.
At first sight, it would seem that white LEDs should make an excellent replacement for the traditional lighting sources. At equal power, they give far more light output than do incandescent bulbs, or, what is the same thing, they use much less power for equal light; and their operational life is orders of magnitude larger, namely, 10-100 thousand hours vs. 1-2 thousand hours. Similarly, their ultimate efficiency is higher than that of fluorescent tubes, and their lifetime is also substantially longer than that of fluorescent tubes.
However, LEDs have a number of drawbacks that have prevented them, so far, from being widely adopted as traditional lighting sources. One of these is that LEDs are discrete sources of light. They produce intense light within the beam of their output, but dim light outside of that beam. Using multiple LEDs does not fully alleviate this problem, as there are then interference patterns in the light.
In the past, LEDs have had diffusers added to their shells or bodies to spread out the light from the LED as a solution to the point light source problem. Another method has been to roughen the surface of the LED package. Neither of these methods accomplishes uniform light distribution for an LED light source, and may lower luminous efficiency. Methods of accomplishing approximate angular uniformity may also involve partially absorptive processes, further lowering luminous efficacy.
Another drawback with LEDs is that although LEDs require substantially less power for a given light output than do incandescent bulbs, it still takes many watts to generate adequate light for illumination. An LED, being a semiconductor, is nearly a point source of heat, and cannot be allowed to get hotter than a range of approximately 85-150° C. The LED thus has a substantial heat problem.
One possible solution to this heat problem is to use a large metallic heat sink, attached to the LEDs. This heat sink would then extend out away from the bulb, removing the heat from the LEDs. However, this solution is undesirable, because the heat sink may make it difficult for the light source to fit in to the desired form factor.